Travelling wave magnetron tube



Nov. 18, 1958 A. LERBS 2,861,212

TRAVELLING WAVE MAGNETRON TUBE Filed July 11, 1952 3 Shets-Sheet 1 Qwmznzmmmzn [NVENTOR ALFRED LERBS AGENTS Nov. 18, 1958 A. LERBS TRAVELLING WAVE MAGNETRON TUBE 3 Sheets-Sheet 2 Filed July 11. 1952 I N V E NTD'R ALFR E) LERBS FY f o.

AQENTS Nov. 18, 1958 'A. LERBS 2,861,212

TRAVELLING WAVE MAGNETRON TUBE Filed July 11, 1952 3 Sheets-Shae; 3

Fm ml Q u 1 0 Q sf m g '0 Q1 p j "1 .3 s k jm v o S s E AGENTS United States Patent TRAVELLING WAVE MAGNETRON TUBE Alfred Lerbs, Paris, France, assignor to Compagnie Generale de Telegraphic Sans Fil, a corporation of France Application July 11, 1952, Serial No. 298,368 Claims priority, application France July 30, 1951 4 Claims. (Cl. 3153.6)

This invention relates to travelling wave magnetron tubes.

A well known form of amplifier or frequency multiplier for very short (decimetre or centimetre) waves is the so-called travelling wave tube. In this form of tube a wave fed into a delay line forming part of the tube is amplified by interaction with an electron beam which is propagated in a direction parallel to said line and perpendicular to crossed electric and magnetic fields, the speed of the beam being substantially equal to the ratio of' these two fields and to the speed of phase propagation of the wave.

The present invention seeks to provide a travelling wave tube of improved efiiciency, and this object is achieved by dividing the delay line into two sections, the first of which is placed in the negative electrode of the interaction space.

According to this invention a travelling wave tube comprising a delay line, means for feeding an electromagnetic wave into said line, and means providing an electron beam moving parallel to said line and perpendicularly to crossed electric and magnetic fields at a speed equal to the ratio of these two fields and to the speed of phase propagation of the wave is characterised in that the delay line is divided into two sections, the first of which constitutes the negative electrode of the beam propagation space, situated opposite a positive electrode having no delaying properties, said positive electrode being in alignment with the second section of the delay line constituting the positive electrode of the second part of the beam propagation space, said second section being situated opposite a negative electrode having no delaying properties and in alignment with said first section of the delay line.

The invention is illustrated in and explained in connection with the accompanying drawings in which Figure 1 is a diagrammatic section illustrating a known amplifier tube of the travelling wave type in question.

Figures 2a, 2b are diagrams explaining the functioning of this tube.

Figures 3a, 3b, 3c, 3d are similar diagrams for a tube the input of "which is coupled to a delay line forming the negative electrode.

Figure 4 is a diagrammatic section of an amplifier or frequency multiplier tube in accordance with this invention.

Figure 5 illustrates an external excitation oscillator using a tube of the same type as Figure 4.

Figure 6 is a cross section of a tube according to any of Figures 1, 4 or 5 showing the pole pieces 11, 12 of the magnetic system for producing the magnetic field.

For the better understanding of the considerations underlying and precedent to the present invention, the electronic mechanism of known travelling wave tubes will first be briefly reviewed with reference to Figure l, which is a diagrammatic section of such a tube, and Figures 2a and 2b which indicate respectively the lines of force "ice and equipotential lines of the electric field in the pres ence of the high frequency wave.

In Figure 1, the electrons of an electronic beam 1 issueing from a cathode 2 enter the interaction space situated between a delay line 3 which is maintained at a positive potential, and a negative electrode 6. The delay line has an input 4 and an output 5 for the high frequency wave. The electrons are first focused in respect ofphase and then yield their continuous potential energy to the high frequency field of the travelling wave. In the course of this yield of energy, the electrons approach the line 3, while in the absence of the high frequency field they are collected by a collector 7. A part of the line 3 carries an attenuator layer '8 in order to avoid self-oscillation. The part 9 of the line functions principally as a phase focuser, and the part. 10

principally as an amplifier. An electric field of value E transverse to the beam and situated in the plane of the figure is established between the electrodes 3 and 6, and a magnetic field of value B related to the electric field E and the speed v of the electrons by the relation is established perpendicularly to the plane of the figure.

In Figure 2a are shown the lines of force of the high frequency field which are displaced in space at the phase speed ofthe wave. Figure 2b shows the equipotential lines of the resultant field. It will be seen from Figure 2a that the transverse component of the high frequency field is subtractive with respect to the D. C. field E at points such as A, C, E and that it is additive thereto at intermediate points such as B, D so that the resultant transverse field becomes alternately weaker and stronger than the D. C. field at the points A, B, C, D and E. If the speed of the beam in the D. C. field is equal to the ratio E /8, the electric force acting on the electrons is balanced by the Lorentz force due to the speed of the electrons and to the magnetic field. The transverse high frequency component, alternately subtracted from and added to the D. C. component, acting jointly with the magnetic field, imparts to the electrons a supplementary speed which is superimposed on the continuous speed and which is such that the complement of the electric force is balanced by the complement of the Lorentz force. The electrons are thus retarded or accelerated depending on their position in relation to the travelling wave, as indicated by the arrows between Figures 2a and 2b. These accelerations and decelerations produce groupings of electrons in the regions symbolised by the circles G, where the transverse high frequency field component is nil and the longitudinal component is opposed to the direction of the beam. It will be seen from Figure 21) that in these regions the equipotential lines of the resulting field are directed obliquely towards the delay line, following the direction of the beam, so that the assembled electrons are subjected to a force which pushes the-m towards the anode without changing the longitudinal speed. They therefore lose a part of their potential energy, which is yielded to the high frequency field.

The electronic efficiency of such a tube becomes higher, the nearerthe electrons are to the cathode potential in the absence of a high frequency field. :But when the high frequency field is then applied, its intensity is low, at a distance from the delay line, and the electrons are in a low high frequency field during the focusing, which is unfavourable to amplification. In the known tube, the conditions of good efiiciency are thus incompatible with the conditions of good amplification.

In order to overcome this difliculty, it has already been proposed to provide the cathode of such a tube with an 3. electron optical system, or to divide the line into two parts, the first of which, intended for the focusing of the electrons, is nearer the negative electrode. These proposals however present various disadvantages in respect of construction and energy, which render them unusable in practice.

The present invention which does not have the disadvantages of the previous proposals referred to, consists in dividing the interaction space into two parts, the energy exchange mechanism in the first part being the inverse of the mechanism explained with the aid of Figures 2a and 2b;

This inverse mechanism takes place in a tube in which the direction of the D. C. field has been reversed in relation to the high frequency'field, that is to say where the delay line has been brought to the negative potential, the positive electrode being planar. Figures 3a and 312 show for this case the images of the lines of force and equipotential lines in similar manner to Figures 2a and 2b. In this case also phase focusing, is obtained at points where the transverse component of the high frequency field is nil. Nevertheless, in contradistinction to Figures 2a and 2b the longitudinal component of the high frequency field at these points is directed in the same direction as the beam. The equipotential lines of the resulting field, in the direction of the beam, are directed obliquely towards the delay line. The electrons are thus pushed towards that negative electrode. During this movement they gain. potential energy, which is yielded to them by the high frequency field of the travelling wave. The energy effect is therefore the inverse of that explained with the aid of Figures 2a and 2b, and the focusing wave, instead of being amplified, is attenuated.

Nevertheless, the images of Figures 3a and 3b are valid only if the focusing line is fairly short, for the focused, beam excites in the delay line a wave the high frequency field of which seeks to brake the electrons for reasons of energy. This field is therefore, when the delay line is negative, in opposition with that of the focusing wave which is attenuated as it travels. If the line is too long, the field of the excited wave becomes predominant in relation to that of the focusing wave, and the resulting high frequency field is inverse in relation to Figures 3a and 3b, that is to say the images shown in Figures 3c and 3d are obtained. It will be seen that the electrons are here again pushed towards the positive electrode, that is to say they lose potential energy at the profit of highfrcquency energy, but that the groupings of electrons are defocused, that is to say that conditions are not favourable to operation.

The present invention therefore divides the delay line into two sections, the first of which, placed in the negative electrode, is relatively short. The effect obtained will then be that illustrated in Figures 3a and 3b, and not that illustrated in Figures 3a and 3d, that is to say the electrons will be strongly focused, with very favourable consequences for the electronic efficiency in the course of normal amplification in the interaction space of the second section. With regard to the attenuation of the wave brought to the input of the first section, it will be made up and even exceeded by the gain realised in the course of the amplification of the second section, so that at the output an improved efficiency will be obtained while losing nothing in respect of amplification as compared with the known tube illustrated in Figure 1.

Figure 4 illustrates diagrammatically a simple embodiment of the invention. The references correspond to those in Figure 1, from which Figure 4 differs only by the fact that the first section 9 is placedin alignment with the negative electrode 6 and brought to the same potential as the latter. It is thus opposite the attenuated section Sconnected to the second positive section 10. The electromagnetic wave enters at 4 and, as has been explained, causes strong focusing of the electrons of the beam 1. The focused electrons penetrate into the interaction space 4 between 6 and 10, and at 10 excite a travelling wave which enters into interaction with the beam as in the known tube illustrated in Figure 1. That is to say the high frequency field of the wave excited in the line 10 continues to focus the electrons, which continually yield a part of their potential energy to'the field of the wave as they approach the line.

The main advantages of this invention are (1) The tube is made relatively short.

(2) There is suppression of any tendency to self-cs cillations on the section 9, even if the latter is not attenuated.

(3) There is very high electronic efficiency, owing to the fact that, through interaction with the longitudinal component of the high frequency field, a part of the electrons gains during focusing potential energy which is supplied by the high frequency field of the focusing wave and this energy is re-transformed into high frequency energy in the amplifier part of the tube.

In order to prevent the electrons in course of focusing from being picked up by the focusing line, the cathode is preferably brought to a slightly more positive potential than the potential of the focusing line.

, advantage of shortening the tube, since the sections 8 and Q are superimposed and not in alignment as in the case of Figure 1. v

The tube illustrated in Figure 4 can also function as a frequency multiplier if the dimensions are made such that the wave excited in the section 10 is of a harmonic frequency in relation to the wave in the section 9, the

phase speeds in the two sections, which are equal to the speed of the beam, corresponding on the one hand to the fundamental wave and on the other hand to the desired harmonic wave.

An arrangement in accordance with the invention may advantageously utilise a delay line propagating energy in the opposite direction to the beam, which, as is well known, is a property characterising for example symmetrical interdigital lines or so-called meander-wound (zig-zag) lines. Such lines are likely to cause selfoscillations if used in known travelling wave tubes, but in arrangements according to this invention, such lines may be used in the focusing section 9 without the danger of self-oscillation and, without having to provide any artificial attenuation means. On the other hand, by using such lines in the amplifier section 10, with the provision of the attenuator 8 at the end on the collector side and of the output 5 on the input side (Figure 5), it is possible to make a self-oscillator controlled by the wave introduced at 4, while the self-oscillation may be arranged to take place at a multiple frequency if the phase speeds of the respective waves on the two sections are suitably adapted to the speed of the beam.

I claim:

1. A traveling wave tube of the type comprising a cathode, a delay line and a smooth electrode, parallel to said delay line and defining therewith an electron and wave interaction space, said cathode being positioned for propagating an electron beam in said space, said tube further comprising an additional delay line portion extending between said cathode and said electrode.

2. A traveling wave tube of the type comprising a cathode, a delay line and a smooth electrode, parallel to said delay line and defining therewith an electron and wave interaction space, said cathode being positioned for propagating an electron beam in said space, said tube furthercomprising an additional delay line portion in alignment with said electrode and extending between said cathode and said electrode.

3. A traveling wave tube of the type comprising a cathode, a delay line and a smooth electrode, parallel to said delay line and defining therewith an electron and wave interaction space, said cathode being positioned for propagating an electron beam in said space, said tube further comprising an additional delay line portion in alignment with said electrode and extending between said cathode and said electrode, said delay line comprising an attenuated part facing said additional delay line portion.

4. A traveling wave tube of the type comprising a cathode, a delay line and a smooth electrode, parallel to said delay line and defining therewith an electron and wave interaction space, said cathode being positioned for propagating an electron beam in said space, said tube further comprising an additional delay line portion in alignment With said electrode and an additional smooth electrode portion in alignment with said delay line, said additional delay line portion extending between said cathode and said electrode, and said additional smooth electrode portion facing said delay line portion over at least a portion thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,414,121 Pierce Jan. 14, 1947 2,511,407 Kleen et a1 June 13, 1950 2,531,972 Doehler et a1 Nov. 28, 1950 2,602,148 Pierce July 1, 1952 2,620,458 Spencer Dec. 2, 1952 2,622,158 Ludi Dec. 16, 1952 2,687,777 Warnecke et al Aug. 31, 1954 Charles Nov. 16, 1954 

